To reduce the generating amount of nitrogen oxide(NOx) by interposing a protective atmosphere between combustion and a charged matter.

Protective gas is provided into a furnace through at least on set of injectors 8 to a position near the upper surface height position 7 of the final flat bath of a charged matter at the position of a height lower than a first vertical distance above a hearth 5 and the upper position of the same. The protective gas forms a protective gas layer 12, including a space in a filler, laminated between the hearth 5 and a combustion layer 6, in the furnace whereby the most of or the whole of the charged matter is protected from combustion reaction products by the protective gas layer 12.